The invention relates to a reflection photometric apparatus for a camera including a focal plane shutter, and more particularly, to such apparatus in which a proper exposure period is automatically determined by the photometry (measurement) of light from an object being photographed which is reflected from either one or both of a shutter blind surface and a film surface.
Since the purpose of a photometric apparatus used in a camera is to determine the amount of light reflected from an object being photographed in order to assure a proper exposure onto a film, it is desirable to effect photometry by disposing a photometric, photoelectric transducer element on a location which is ideally on a film surface, or on an equivalent surface, for example, on a shutter blind surface of a focal plane shutter. However, this is impossible in practice, and the usual practice is to employ a so-called direct photometry system in which light from an object being photographed which is reflected by a film surface or its equivalent shutter blind surface is determined. As is well recognized, when a blind shutter of the focal plane type is used, the film surface is initially covered by a first shutter blind which is formed by a black cloth. As the first blind begins to run is response to a shutter release operation, the film surface begins to be exposed in accordance with the movement of the blind across the picture frame. After a given time interval has passed which corresponds to a proper exposure period, a second shutter blind which is again formed by a black cloth is allowed to run, again covering the exposed film surface.
It will also be noted that for a high speed shutter operation, the second blind is allowed to run during the time the first shutter blind continues to run so that a reduced exposure period can be obtained. It will be understood that as the exposure period is reduced, the width of a slit formed between the first and the second shutter blind also decreases.
When the timing of a proper exposure is determined by the photometry of light from an object being photographed which is reflected by the first shutter blind and the film surface, which are interrelated as mentioned above, the reflected light from the first shutter blind is initially determined, followed by the determination of reflected light from the film surface being exposed. In this instance, since the blind surface and the film surface have substantially different optical reflectivities, some compensation must be made. Specifically, if the reflectivity of the first blind surface is different from that of the film surface, the photometric value which is obtained by the photometry of respective reflected light will be calculated at different rates, preventing a proper exposure from being achieved.
For this reason, in a conventional photometric apparatus of this kind, a material which exhibits the same reflectivity as the film surface has been printed on the first shutter blind in a pattern in order to achieve the same reflectivity for both the first shutter blind surface and the film surface. However, a shutter blind comprises a cloth located on the front side and which is lined with rubber fabric on the rear side, and therefore it is very difficult to work the front side as by printing, resulting in a very expensive structure. In addition, the arrangement suffers from difficulties that a change in the reflectivity occurs if the printed pattern varies from blind to blind and that the printed pattern may be exfoliated and the planarity of the blind surface may be degraded since the shutter blind is wound up by a rapid operation. Furthermore, the printing of reflecting patterns on the first shutter blind may interfere with a dimming effect of black delustering paints which are provided on the internal surfaces of the camera in order to reduce stray light within mirror box, leakage of light onto the film or to prevent ghost or flare. Thus, the printing may result in the occurrence of flare or ghost.
To eliminate such disadvantages, there is proposed an arrangement in which a non-worked, black first shutter blind is directly used without a pattern printing, and a proper exposure is achieved by compensating for differential optical reflectivities of the first shutter blind and the film surface.
FIG. 1 illustrates how a film surface which has been covered by a first shutter blind begins to be exposed as the first blind runs. In FIG. 1, a rectangular frame F represents a picture frame associated with the film, and as the first shutter blind B moves in a direction indicated by an arrow a.sub.0, a film surface E which has been initially covered by the first blind begins to be exposed in a sequential manner. The abscissa represents time t. Specifically, when the first blind B leaves the left-hand edge of the frame F at time t=T.sub.1, the exposure of the film surface E is initiated. At time t=T.sub.2, the first blind B passes through the right-hand edge of the picture frame F, fully exposing the picture frame. In FIG. 1, the first blind B has partially exposed the film surface E. Representing the exposed area of the film surface E by A.sub.2, and the area thereof which is still covered by the first blind B by A.sub.1, the total area A.sub.0 of the picture frame F is equal to the sum of A.sub.1 and A.sub.2. The time variation of these areas is illustrated in FIG. 2. Thus for t&lt;T.sub.1, the exposed surface areas A.sub.2 of the film surface E remains zero while the area A.sub.1 is equal to the total area A.sub.0 of the picture frame F which is entirely covered by the first blind B. When the time t is between the times T.sub.1 and T.sub.2, the exposed area A.sub.2 of the film surface E increases with time t while the area A.sub.1 of the film surface E which is covered by the first blind B decreases with time t. However, the sum of A.sub.1 and A.sub.2 remains equal to the total area A.sub.0. For t&gt;T.sub.2, the area A.sub.2 of the film surface E is equal to the total area A.sub.0 while the area A.sub.1 of the film surface E which is covered by the first blind B is zero.
In a method of achieving a proper exposure by the photometry of reflected light from the first shutter blind and the film surface, one of which is moving relative to the other and having varying areas, the first blind should ideally be removed so that the photometry is made only for the reflected light from the film surface. However, as a matter of practice, there exists the first blind, and hence by determining the reflected light from the first blind surface, a difference cover the reflected light from the film surface is calculated to compensate for the difference, thereby enabling a proper exposure.
When determining the reflected light from the first shutter blind surface and the film surface, a photoelectric transducer element such as silicon photodiode is usually used to convert the light input into a corresponding photocurrent, which then charges a capacitor to develop an integrated voltage thereacross. The integrated voltage is compared against a given decision level, thereby determining a proper exposure period.
FIG. 3 graphically illustrates an exemplary photocurrent I.sub.P taken on the ordinate and plotted over the time t on the abscissa. As will be seen, if the first shutter blind surface has the same reflectivity as the film surface, the photocurrent will not change over time or as the first blind moves, providing a constant photocurrent I.sub.F which corresponds to the reflectivity of the film surface. However, when the first shutter blind surface has a different reflectivity from the film surface, the photocurrent I.sub.P will change with the movement of the first blind. In FIG. 3, times T.sub.1, T.sub.2 represent the times corresponding to those times shown in FIG. 1. Thus, the exposure of the film surface is initiated at time T.sub.1 and the first blind has fully exposed the picture frame at time T.sub.2. An intermediate time when the first blind is moving across the film surface is designated by T.sub.C. The photocurrent I.sub.S shown on the ordinate represents a photocurrent which results from the reflected light from a black first shutter blind surface which is not provided with a printed pattern. The photocurrent I.sub.F represents a photocurrent which results from the reflected light from the film surface.
In FIG. 3, curves I.sub.A, I.sub.AR and I.sub.B represent varying photocurrents which result from reflected light from a black first blind surface which is not provided with the printed pattern. Specifically, the curve I.sub.A depicts a varying photocurrent when the photometry is effected with a photoelectric transducer element having a center concentrated orientation characteristic. Thus, until time T.sub.C when the first blind moves past the central region of the film surface, the photocurrent from the transducer element is equal to the photocurrent I.sub.S that is due to the first blind surface while after time T.sub.C, it is equal to the photocurrent I.sub.F due to the film surface. It should be noted that the curve I.sub.A is depicted theoretically only, and it will be noted that it sharply rises at right angles at time T.sub.C. However, even a transducer element having a center concentrated orientation characteristic has a certain degree of distributed sensitivity about the center, so that the actual change of the photocurrent will be as shown by the curve I.sub.AR. Thus, although the photocurrent exhibits a point of inflection at time T.sub.C, the change is not that of right angles, but occurs gradually around the time T.sub.C.
The curve I.sub.B represents a change of a photocurrent which is obtained with a photoelectric transducer element having a uniform orientation characteristic. The photocurrent increases in proportion to the running of the first blind which causes the film surface to be exposed. It will be noted that the curve I.sub.B is similar to the change of the area A.sub.2 of the film surface shown in FIG. 2. Thus, the photocurrent is equal to the photocurrent I.sub.S due to the reflection from the first blind surface prior to time T.sub.1, and is equal to the photocurrent I.sub.F due to the reflection from the film surface after time T.sub.2.
FIG. 4 represents an integrated voltage which is produced by the photocurrent I.sub.P. The integrated voltage V.sub.C is shown on the ordinate while the abscissa represents the time t. The straight line F.sub.V represents an ideal integrated voltage, plotted over the time, which results from the reflected light from the blind surface having the same reflectivity as the film surface. By contrast, curves A.sub.V, B.sub.V represent the integrated voltages which result from the reflected light from a black first blind surface which is not provided with a printed pattern. These curves correspond to the curves I.sub.A, I.sub.B of FIG. 3. Specifically, the curve A.sub.V represents an integrated voltage obtained by the photometry with a photoelectric transducer element having a center concentrated orientation characteristic while the curve B.sub.V represents an integrated voltage obtained by the photometry with a photoelectric transducer element having a uniform orientation characteristic.
Time T.sub.1 on the abscissa represents the initiation of exposure of the film surface while time T.sub.2 represents the time when the first blind has fully exposed the picture frame. Time T.sub.3 indicates the initiation of running of the second blind after a proper exposure has been given. Times T.sub.4a, T.sub.4b represent the times when the integrated voltage which result from the reflected light from the black first blind surface becomes equal to a decision level V.sub.COM, to be described later, when transducer elements having the center concentrated orientation characteristic and the uniform orientation characteristic, respectively, are used. It is to be noted that time T.sub.C represents the time when the trailing edge of the first blind moves past the center of the film surface. As will be evident, the integrated voltage is compared against the decision level V.sub.COM, and allows the second shutter blind to start running to close the shutter when the integrated voltage becomes equal to the decision level.
Considering the curve A.sub.V more specifically, it is noted that it presents an integrated voltage which is reduced in comparison to that of the ideal line F.sub.V and presents a substantial time lag with respect thereto. The curve A.sub.V crosses the decision level V.sub.COM at point P.sub.2 or at time T.sub.4a, which has a time lag of T.sub.4a -T.sub.3 with respect to the point P.sub.F where the line F.sub.V crosses the decision level V.sub.COM. The time lag represents an error in the exposure period. The curve A.sub.V has a bend at point P.sub.1, and thus it comprises a pair of line segments P.sub.0 -P.sub.1 and P.sub.1 -P.sub.2. The point P.sub.1 will be referred to as a break point. Since the curve A.sub.V represents an integration of a photocurrent which is represented by the curve I.sub.A, the integrated voltage is linear and low, as indicated by the line segment P.sub.0 -P.sub.1, inasmuch as the associated transducer element receives the reduced reflected light from the first blind surface and provides the photocurrent I.sub.S of FIG. 3 until time T.sub.C when the trailing edge of the first blind has moved past the center of the film. After T.sub.C, the transducer element receives the reflected light from the film surface or provides the photocurrent I.sub.F, and hence the integrated voltage increases in parallel relationship with the line F.sub.V, as indicated by the line segment P.sub.1 -P.sub.2. Thus, the curve A.sub.V comprises the line segment P.sub.0 -P.sub.1 for 0&lt;t&lt;T.sub.C and another line segment P.sub.1 -P.sub.2 for time t&gt;T.sub.C, which are joined together at the break point P.sub.1. On the other hand, the curve B.sub.V exhibits the similar integrated voltage as the curve A.sub.V until point P.sub.3 corresponding to time T.sub.1, but when the film surface begins to be exposed at time T.sub.1, the reflected light increases gradually, and after point P.sub.4 corresponding to the time T.sub.2 when the picture frame is fully exposed, the reflected light is entirely due to the reflection from the film surface, whereby the curve A.sub.V becomes parallel to the line F.sub.V. The curve B.sub.V crosses the decision level V.sub.COM at point P.sub.5 corresponding to time T.sub.4b, which has a time lag of T.sub.4b -T.sub.3 with respect to time T.sub.3 when the line F.sub.V crosses the decision level V.sub.COM. Again, the time lag represents an error in the exposure period. A curve shown in broken lines AR corresponds to the curve I.sub.AR shown in FIG. 3, and is offset from the curve A.sub.V adjacent to the break point P.sub.1 in the similar manner as the curve I.sub.AR is slightly offset from the ideal curve I.sub.A.
It will be seen that the reflected light from the film surface and the first blind surface, formed by a black cloth, depends on the respective reflectivities, and accordingly the photocurrent which results from the reflected light from the film surface having an increased reflectivity is high while the photocurrent which results from the reflected light from the first blind surface having a reduced reflectivity is low. Consequently, the integrated voltages, which represent an integration of the respective photocurrents, are as shown in FIG. 4.
When the photoelectric transducer element is used for photometry which provides a curve similar to the curve F.sub.V shown in FIG. 4 or which receives reflected light from the film surface or a blind surface having an equivalent reflectivity, a proper exposure period can be correctly determined. However, when the photometry is made for the reflected light from a combination of the first blind surface, such as a black cloth, and the film surface, as would be illustrated by the curves A.sub.V, B.sub.V, the resulting integrated voltage is reduced as compared with that of the ideal curve F.sub.V, so that a proper exposure period cannot be obtained.
To overcome this difficulty, there has been proposed to provide means which corrects an integrated voltage as typically exemplified by the curve A.sub.V or B.sub.V in order that a proper exposure period may be determined. By way of example, in Japanese Patent Application No. 27,848/1979, corresponding to U.S. Application Ser. No. 109,762, now U.S. Pat. No. 4,295,750, and West German Patent Application No. P.30 08 864.3, there is disclosed correction means in the form of a capacitor switching system which is used for a zero bias, direct integrating arrangement. In another Japanese Patent Application No. 27,847/1979, there is disclosed correction means in which the amplification factor of a variable amplification arithmetic circuit is varied in a time sharing technique to correct the integrated voltage.
However, the former arrangement suffers from the disadvantages that because a plurality of integrating and correcting capacitors are used, the implementation of the circuit into an integrated circuit is difficult to achieve, and because the circuit becomes an extensive one, a satisfactory functioning of an analog switch cannot be provided, resulting in a failure to provide a sufficient correction. The latter arrangement involves a sequential switching of a number of analog switches. This results in an increased number of inflection points as the switching takes place, thereby disadvantageously causing a hunting of the shutter at these points.